A polymer film of typically cellulose acylate, polyester, polycarbonate, cycloolefin polymer, vinyl polymer, polyimide and the like is used in silver halide photographic materials, optical compensatory films, polarizers and liquid-crystal display devices. From these polymers, films having more excellent surface smoothness and uniformity can be produced, and therefore the films are widely employed for optical applications.
Of those, a cellulose ester film having a higher water vapor permeability, as compared with other polymer films, can be directly stuck to a most popular polarizing film of polyvinyl alcohol (PVA)/iodine, in on-line operation. Accordingly, cellulose acylate, especially cellulose acetate and cellulose acetate propionate are widely employed for a protective film for polarizers.
On the other hand, however, a polarizing film of polyvinyl alcohol has poor resistance to moisture and therefore water application to the polarizing film may have negative influences on the film in that the degree of polarization of the film may be lowered. Accordingly, the protective film of a polarizer that comprises the polarizing film is required to have a low water vapor permeability.
To satisfy the above-mentioned two requirements, a method has been tried for lowering the water vapor permeability of cellulose acylate film. Concretely, the method comprises adding a hydrophobic additive to cellulose acylate film (e.g., JP-A-2002-146044 and JP-A-2005-89680). However, this is problematic in that, when a hydrophobic additive is added to a hydrophilic cellulose acylate film and when the film is put under wet heat, then the additive may be expelled from the film with the result that the water vapor permeability of the film is to increase.